Method of making a lithographic printing plate precursor

ABSTRACT

A method of making a heat-sensitive lithographic printing plate precursor is disclosed which comprises the steps of (i) providing a web of a lithographic support having a hydrophilic surface; (ii) applying on the hydrophilic surface of the web a coating comprising a phenolic resin; (iii) drying the coating by supplying heat to the coated web; (iv) a cooling step wherein the web temperature is reduced at an average cooling rate which is higher than if the web would be kept under ambient conditions but not higher than 30° C./s; and (v) winding the precursor on a core or cutting the precursor into sheets. The cooling step provides a significant improvement of the aging behavior of the precursor. A stable sensitivity is obtained shortly after coating.

FIELD OF THE INVENTION

The present invention relates to a heat-sensitive lithographic printingplate precursor that comprises a phenolic resin.

BACKGROUND OF THE INVENTION

Lithographic printing typically involves the use of a so-called printingmaster such as a printing plate which is mounted on a cylinder of arotary printing press. The master carries a lithographic image on itssurface and a print is obtained by applying ink to said image and thentransferring the ink from the master onto a receiver material, which istypically paper. In conventional lithographic printing, ink as well asan aqueous fountain solution (also called dampening liquid) are suppliedto the lithographic image which consists of oleophilic (or hydrophobic,i.e. ink-accepting, water-repelling) areas as well as hydrophilic (oroleophobic, i.e. water-accepting, ink-repelling) areas. In so-calleddriographic printing, the lithographic image consists of ink-acceptingand ink-abhesive (ink-repelling) areas and during driographic printing,only ink is supplied to the master.

Printing masters are generally obtained by the image-wise exposure andprocessing of an imaging material called plate precursor. In addition tothe well known photosensitive, so-called pre-sensitized plates, whichare suitable for UV contact exposure through a film mask, alsoheat-sensitive printing plate precursors have become very popular in thelate 1990s. Such thermal materials offer the advantage of daylightstability and are especially used in the so-called computer-to-platemethod wherein the plate precursor is directly exposed, i.e. without theuse of a film mask. The material is exposed to heat or to infrared lightand the generated heat triggers a (physico-)chemical process, such asablation, polymerization, insolubilization by crosslinking of a polymer,heat-induced solubilization, or particle coagulation of a thermoplasticpolymer latex.

Although some of these thermal processes enable plate making without wetprocessing, the most popular thermal plates form an image by aheat-induced solubility difference in an alkaline developer betweenexposed and non-exposed areas of the coating. The coating typicallycomprises an oleophilic binder, e.g. a phenolic resin, of which thedeveloper solubility is either reduced (negative working) or increased(positive working) by the image-wise exposure. During processing, thesolubility differential leads to the removal of the non-image(non-printing) areas of the coating, thereby revealing the hydrophilicsupport, while the image (printing) areas of the coating remain on thesupport. Typical examples of such plates are described in EP-As 625728,823327, 825927, 864420, 894622 and 901902.

The industrial manufacturing of printing plate precursors involves thesteps of unwinding a coil of the support material in web form which istypically aluminum, coating one or more layers on the web, drying thecoating by blowing hot air on the web and finally rewinding the coatedweb on a core or immediately cutting the coated web in sheets which arethen stacked and packaged. On an industrial scale, all these steps arecarried out “on-line”, i.e. on a moving web in a single continuousoperation without any intermediate storage.

A specific problem associated with thermal plate precursors comprisingphenolic resins is that their sensitivity is not stable over timebecause the coating gradually becomes more resistant against thedeveloper and therefore more heat needs to be applied during theimage-wise exposure for triggering the imaging mechanism. Typically ahigh sensitivity, e.g. less than 100 mJ/cm², is obtained just aftercoating and then slowly decreases to reach an equilibrium value of e.g.250 mJ/cm². The aging period that is required to arrive at a stablesensitivity may take several months after coating. In order to reducethe aging period, WO 99/21715 proposes a heat treatment by leaving thematerial shortly after coating in an oven at 40 to 90° C. for anextended period, which is at least 4 hours and most preferably at least48 hours. U.S. Pat. No. 6,251,559 disclosed that a controlled slowcooling after the heat treatment provides additional improvements.According to the latter document, “controlled slow cooling” means thatheat is lost from the precursor more slowly than if it is cooled underambient conditions. Examples of such a cooling method include insulatingthe material after the heat treatment or leaving it in an oven whichprogressively cools to lower temperature. Such a cooling process lastsseveral hours and can only be carried out “off-line”, i.e. a coil or astack of sheets is placed in an oven and left there during the requiredtime. Off-line storage however is to be avoided for several reasons.Besides additional cost and logistic implications, it is quite clearthat a coil or stack cannot be cooled uniformly since the interior ofthe coil or stack will go through a different temperature profile thanthe exterior. Therefore, there is a need for a method that provides aneffective cooling step which can be implemented on-line, before windingthe web on a coil or cutting the web into sheets.

SUMMARY OF THE INVENTION

It is an aspect of the present invention to provide an on-line methodfor aging heat-sensitive printing plate materials which contain aphenolic resin. This object is realized by the method of claim 1, havingthe characterizing feature that the dried coating is subjected to anon-line cooling step. According to the present invention, the longcooling processes which have been disclosed in the prior art and whichcan only be carried out off-line, are replaced by an on-line coolingstep wherein the web temperature is reduced at an average cooling ratewhich is higher than if the web would be kept under ambient conditionsbut not higher than 30° C./s.

The method of the present invention allows to manufacture heat-sensitiveprinting plate precursors having a stable sensitivity within a couple ofweeks instead of several months after manufacturing. No additional agingis required, but it is self evident that embodiments wherein an on-linecooling step according to the present invention is combined with anadditional off-line cooling step, are nevertheless within the scope ofthe present invention.

Specific embodiments of the invention are defined in the dependentclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the web temperature profile during a preferred method ofmaking a heat-sensitive lithographic printing plate precursor accordingto the present invention.

FIG. 2 shows a schematic representation of an apparatus for performing asuitable example of the method of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The heat-sensitive lithographic printing plate precursor of the presentinvention contains a hydrophilic support and a coating comprising aphenolic resin provided thereon. The coating may consist of one or morelayer(s) of which examples are discussed below. The phenolic resin canbe present in one or more layers of said coating.

Any reference herein to the temperature of the precursor is consideredas a reference to the temperature of the support as well as of thecoating: typically, the coating is very thin, in the order of magnitudeof one or a few micrometer, whereas the support has a typical thicknessof between 0.1 and 0.5 millimeter; therefore the support, which ispreferably a metal support, acts as a large heat sink towards thecoating and the temperature of the coating is equal to or very close tothe temperature of the support, irrespective whether the heating andcooling steps discussed herein are carried out by supplying heat or coldto the coated side or the back side of the precursor, or both. Inpractice, temperature values reported herein have been recorded byattaching a thermocouple device, which can be read out remotely, to theback side of the web as it moves through all the sections of the coatingfacility. In that way, a precise temperature profile can be recordedduring all steps of the method of the present invention. Unlessspecifically defined otherwise, all temperatures reported herein are webtemperatures obtained from said thermocouple. In view of the aboveconsiderations, it is clear to the skilled person that the webtemperature value is essentially equal to the temperature of the drycoating that is provided on the web.

Any coating method can be used for applying one or more coatingsolutions to the hydrophilic surface of the support. A multi-layercoating can be applied by coating/drying each layer consecutively or bythe simultaneous coating of several coating solutions at once. Drying istypically carried out by blowing hot air onto the coating, typically ata temperature of at least 70° C., suitably 80–150° C. and especially90–140° C. Also other heat sources, e.g. infrared lamps or microwaveradiation, can be used in the drying step. The drying time may typicallybe 15–600 seconds. However it is not necessary (and may not even bepossible) to remove all the solvent in the drying step. Indeed theresidual solvent content may be regarded as an additional compositionvariable by means of which the coating composition may be optimized. Theend of the drying step is therefore defined herein as the moment atwhich the coating becomes self-supporting and dry to the touch.

After the end of the drying step, the precursor is preferably subjectedto a short on-line additional heating step. Alternatively, the precursormay first be allowed to cool between the drying and the heating step butthis is not required. During the optional heating step, heat is suppliedto the dry coating so that the temperature of the coating is maintainedat a higher value than if the precursor would be kept under ambientconditions (the temperature of the ambient air is set herein bydefinition at 20° C.). So the temperature of the precursor during theheating step can be lower than at the end of the drying step. Morepreferred, the temperature of the coating during the heating step ismaintained at a value which is higher than the temperature of thecoating at the end of the drying step. Alternatively, the dried coatingcan be heat-treated by extending the length of the drier so thatadditional drying air is blown onto the coating after it has become dry.In that embodiment, the temperature of the coating during the heattreatment may be the same as the temperature at the end of the dryingstep.

During the on-line heating step, the web temperature of the precursor ispreferably increased above the glass transition temperature Tg of thephenolic resin. “Tg” referred to herein is the glass transitiontemperature of the phenolic resin in the composition as it has beencoated, dried and heated, i.e. the glass transition temperature of thecoating comprising the phenolic resin. Said Tg value can readily bemeasured by the known calorimetric methods. The heat treatment issufficiently short so that it can be implemented on-line, e.g. between0.1 and 60 seconds, more preferably from 1 to 30 seconds. Preferably,the web temperature is increased during the heating step up to at least150° C., preferably at least 170° C. The upper web temperature limitduring the heating step is defined by the temperature threshold that isrequired to trigger the imaging mechanism of the coating. Said upperlimit is therefore dependent on the particular composition of thecoating, but is typically about 200° C. or more preferably about 250° C.Heating at still higher temperature may induce irreversible chemical orphysical changes in the coating which would render the precursorunsuitable for image recording.

Heating can be carried out e.g. by blowing hot air and/or steam onto thelithographic printing plate precursor, by irradiating the precursor withinfrared light or microwaves, or by contacting the precursor with aheated roller. Combinations of these methods are also suitable. The hotair and/or steam preferably has a temperature of more than 150° C., morepreferably at least 170° C. The infrared light may irradiate thecoating, the back side of the support or both. If the infrared lightirradiates the coating, then it has a wavelength and/or intensity whichdoes not trigger the imaging mechanism of the coating. The heatedroller, which is preferably thermostatically controlled, may likewise becontacted against the coating, the back side of the support or both, theback side being preferred. The roller is preferably a metal roller.

After the heating step, the precursor is cooled before being wound on acore or cut in individual sheets. The cooling step is a fast, “active”cooling step, i.e. it reduces the temperature of the coating at a highercooling rate than if the precursor would be kept under ambientconditions. So the cooling step, referred to herein, is defined as thestep between the start and the end of the active cooling. In preferredembodiments, further discussed below, the cooling step is a multi-phaseprocess wherein the active cooling can be interrupted by a “passive”cooling phase, typically in the transition of the temperature intervalaround Tg. By “passive” cooling is meant a cooling step during which theweb is cooled at an average cooling rate which is lower than or equal tothe cooling rate obtained if the precursor would be kept under ambientconditions. So the cooling step in the method of the present inventioncan be a sequence of one or more active and passive cooling phases. Insuch multi-phase cooling processes, the active cooling step is definedas the process between the beginning of the first active cooling phaseand the end of the last active cooling phase.

The average cooling rate during the cooling step or during a coolingphase is defined as the ratio of the temperature difference between thebeginning and end of the cooling step or phase and the duration of saidcooling step or phase.

Active cooling can be obtained by various means, e.g. by contacting theprecursor against one or more roller(s), preferably metal roller(s) sothat the heat of the precursor is readily transferred to the roller(s).Other cooling methods are of course also possible, e.g. by blowing aironto the precursor. The use of a metal cooling roller is howeverpreferred because, due to the intimate contact between the coolingroller and the precursor, a temperature decrease which is faster than ifthe precursor would be kept in ambient conditions, i.e. without contactwith a cooling roller, can be induced even if the temperature of thecooling roller is maintained at a value which is higher than thetemperature of the ambient air. So active cooling can be obtained bycontacting the precursor, just after the heating step, against a metalcooling roller which has a temperature of e.g. 50 to 120° C. Coolingrollers consisting of other materials, e.g. with a lower heat-capacityor heat-conductivity can also be used. The cooling roller can becontacted against the back side or the coated side of the web, or both.It is quite clear that a faster cooling effect is obtained if thetemperature difference between the cooling roller and the precursor ishigher. A preferred minimum value of the average cooling rate is 0.5°C./s, more preferably 1° C./s and even more preferably 3° C./s.

It is been established by the inventors that an improved aging behaviorcan be obtained by limiting the average cooling rate to a value of notmore than 30° C./s, more preferably not more than 20° C./s and mostpreferably not more than 10° C./s. The reason therefore probably isrelated to the high content of amorphous state if the phenolic resin israpidly cooled below its Tg. If the coating comprises a high amount ofphenolic resin in the amorphous state, the relaxation to a morecrystalline state which inevitably occurs in the days or weeks after thecoating then could explain the shift towards lower sensitivity that canbe observed during the aging of the material.

On the other hand, a shorter, faster cooling step is preferred in viewof the high speed at which the web is traveling through modern coatingfacilities because otherwise the duration of the cooling step wouldextend the length of the coating alley too much. The best compromisebetween these apparently contradictory requirements can be obtained by athree-phase cooling step as follows:

-   -   cooling phase 1: rapid cooling to decrease the temperature of        the precursor down to a value T1, which is higher than Tg of the        phenolic resin.    -   cooling phase 2: slower cooling to decrease the temperature of        the precursor to a value T2 below Tg.    -   cooling phase 3: again rapid cooling down to about ambient        temperature.

The first rapid cooling phases may involve a very high average coolingrate, e.g. at least 10° C./s, more preferably 10 to 20° C./s or evenmore than 20° C./s. In the second cooling phase, the transition of thetemperature interval around Tg is made at a low average cooling rate,i.e. the web temperature of the precursor is reduced in the interval T1to T2 at an average cooling rate which is lower than in phase 1, e.g.lower than 10° C./s. Preferred values of T1 and T2 are Tg+20° C. andTg−20° C. respectively, more preferably Tg+10° C. and Tg−10° C.respectively. According to an even more preferred embodiment, the rapidcooling in phase 1 progresses until the temperature of the precursor isjust above Tg of the phenolic resin, then a slow cooling is set in fromjust above Tg to just below Tg and then, finally, another rapid coolingphase can be applied without inducing a significant impact on the agingbehavior. The range between “just above” and “just below” Tg as usedherein is e.g. the range from Tg+5° C. to Tg−5° C., more preferably fromTg+2° C. to Tg−2° C.

The average cooling rate in the second cooling phase may be higher orlower than the cooling rate corresponding to ambient conditions, i.e.without the use of cooling means such as a roller. A preferred averagecooling rate in the second cooling phase ranges from 0.1° C./s to 5°C./s, more preferably 0.2° C./s to 3° C./s; values between 1° C./s and2° C./s produce excellent results. Once the web temperature has beendecreased until below Tg in that way, again a rapid cooling can beapplied in the third cooling phase, e.g. at an average cooling rate ofat least 10° C./s, more preferably 10 to 20° C./s or even more than 20°C./s.

Phenolic resins such as the commercially available novolacs have atypical Tg between 75 and 95° C., more typically between 80 and 90° C. Atypical example of a preferred web temperature profile according to theinvention is shown in FIG. 1, wherein the Tg of the phenolic resin is84° C. In FIG. 1, the drying was carried out with hot air having atemperature of 130° C. and hot air at 160° C. was used for the heatingstep. During the first cooling phase, a rapid cooling was obtainedfrom >150° C. down to 100° C. in a few seconds, followed by a slowercooling from 100° to 70° C. in a period of 16 seconds (i.e. at anaverage cooling rate of 1.9° C./s) and finally again a rapid coolingphase to reach about ambient temperature in a few seconds.

The above described heating and cooling steps provide a material whichis characterized by a stable sensitivity after an aging period which issignificantly shorter than if the material has not been subjected tothese steps, e.g. a couple of weeks compared to several months. Inaddition to the improved aging behavior, the coating of materialsaccording to the invention also show a significant improvement of theresistance towards mechanical damage. More particularly, the rubresistance is highly enhanced by the above described cooling processwherein the interval around Tg is passed slowly.

The methods of the present invention can be carried out in a coatingfacility of which a typical example is shown in FIG. 2. The support 1 isunwound from a coil 2, then applied with one or more layers with coater3, the coating is subsequently dried in a multi-section drier 4-5-6-7,heat-treated by heat source 8, which is e.g. an infrared light source ora nozzle blowing hot air, then cooled by roller 9 and finally wound upon core 13. Air nozzles 10-11-12 can be used for additional cooling:roller 9 is preferably maintained at a temperature just above Tg of thephenolic resin and nozzle 10 just below Tg so that the transition of thetemperature interval around Tg is slow.

The formation of the lithographic image by the plate precursor of thepresent invention is due to a heat-induced solubility differential ofone or more layers of the coating during processing in the developer.Typically, the developer solubility of the layer comprising the phenolicresin is changed by the exposure. One or more additional layer(s) maycontribute to the imaging process. In some embodiments, the coating mayfurther comprise layer(s) which do not contribute to the imagingmechanism, e.g. a layer of which the solubility in the developer doesnot substantially change upon exposure. An example thereof is aprotective layer which is provided at the top of the coating and whichmay dissolve in the developer at both exposed and non-exposed areas.Also layers which are provided between the support and the image-forminglayers are typically not contributing to the imaging process.

The solubility differentiation between image (printing, oleophilic) andnon-image (non-printing, hydrophilic) areas of the lithographic image ischaracterized by a kinetic rather than a thermodynamic effect, i.e. thenon-image areas are characterized by a faster dissolution in thedeveloper than the image-areas. In a most preferred embodiment, thenon-image areas dissolve completely in the developer before the imageareas are attacked so that the latter are characterized by sharp edgesand high ink-acceptance. The time difference between completion of thedissolution of the non-image areas and the onset of the dissolution ofthe image areas is preferably longer than 10 seconds, more preferablylonger than 20 seconds and most preferably longer than 60 seconds,thereby offering a wide development latitude.

According to one embodiment, the printing plate precursor isnegative-working, i.e. the image areas correspond to the exposed areas.A suitable negative-working coating comprises a phenolic resin and alatent Brönsted acid which produces acid upon heating or IR radiation.These acids catalyze crosslinking of the coating in a post-exposureheating step and thus hardening of the exposed regions. Accordingly, thenon-exposed regions can be washed away by a developer to reveal thehydrophilic substrate underneath. For a more detailed description ofsuch a negative-working printing plate precursor we refer to U.S. Pat.No. 6,255,042 and U.S. Pat. No. 6,063,544 and to references cited inthese documents.

According to another embodiment, the printing plate precursor ispositive-working. In such an embodiment, one or more layers of thecoating are capable of heat-induced solubilization, i.e. they areresistant to the developer and ink-accepting in the non-exposed stateand become soluble in the developer upon exposure to heat or infraredlight to such an extent that the hydrophilic surface of the support isrevealed thereby. So after exposure and development, the exposed areasare removed from the support and define hydrophilic, non-image(non-printing) areas, whereas the unexposed areas are not removed fromthe support and define an oleophilic image (printing) area.

The support of the lithographic printing plate precursor has ahydrophilic surface or is provided with a hydrophilic layer. The supportmay be a sheet-like material such as a plate or it may be a cylindricalelement such as a sleeve which can be slid around a print cylinder of aprinting press. Preferably, the support is a metal support such asaluminum or stainless steel. The support can also be a laminatecomprising an aluminum foil and a plastic layer, e.g. polyester film.

A particularly preferred lithographic support is an electrochemicallygrained and anodized aluminum support. Graining and anodization ofaluminum is well known in the art. The anodized aluminum support may betreated to improve the hydrophilic properties of its surface. Forexample, the aluminum support may be silicated by treating its surfacewith a sodium silicate solution at elevated temperature, e.g. 95° C.Alternatively, a phosphate treatment may be applied which involvestreating the aluminum oxide surface with a phosphate solution that mayfurther contain an inorganic fluoride. Further, the aluminum oxidesurface may be rinsed with a citric acid or citrate solution. Thistreatment may be carried out at room temperature or may be carried outat a slightly elevated temperature of about 30 to 50° C. A furtherinteresting treatment involves rinsing the aluminum oxide surface with abicarbonate solution. Still further, the aluminum oxide surface may betreated with polyvinylphosphonic acid, polyvinylmethylphosphonic acid,phosphoric acid esters of polyvinyl alcohol, polyvinylsulfonic acid,polyvinylbenzenesulfonic acid, sulfuric acid esters of polyvinylalcohol, and acetals of polyvinyl alcohols formed by reaction with asulfonated aliphatic aldehyde It is further evident that one or more ofthese post treatments may be carried out alone or in combination. Moredetailed descriptions of these treatments are given in GB-A-1 084 070,DE-A-4 423 140, DE-A-4 417 907, EP-A-659 909, EP-A-537 633, DE-A-4 001466, EP-A-292 801, EP-A-291 760 and U.S. Pat. No. 4,458,005.

According to another embodiment, the support can also be a flexiblesupport, which is provided with a hydrophilic layer, hereinafter called‘base layer’. The flexible support is e.g. paper, plastic film, thinaluminum or a laminate thereof. Preferred examples of plastic film arepolyethylene terephthalate film, polyethylene naphthalate film,cellulose acetate film, polystyrene film, polycarbonate film, etc. Theplastic film support may be opaque or transparent. The base layer ispreferably a cross-linked hydrophilic layer obtained from a hydrophilicbinder cross-linked with a hardening agent such as formaldehyde,glyoxal, polyisocyanate or a hydrolyzed tetra-alkylorthosilicate.Particular examples of suitable hydrophilic base layers for use inaccordance with the present invention are disclosed in EP-A-601 240,GB-P-1 419 512, FR-P-2 300 354, U.S. Pat. No. 3,971,660, and U.S. Pat.No. 4,284,705.

The phenolic resin is preferably a binder having acidic groups with apKa of less than 13 to ensure that it is soluble or at least swellablein aqueous alkaline developers. Advantageously, the binder is a polymeror polycondensate having free phenolic hydroxyl groups, as obtained, forexample, by reacting phenol, resorcinol, a cresol, a xylenol or atrimethylphenol with aldehydes, especially formaldehyde, or ketones. Thepolymers may additionally contain units of other monomers which have noacidic units. Such units include vinylaromatics, methyl (meth)acrylate,phenyl(meth)acrylate, benzyl (meth)acrylate, methacrylamide oracrylonitrile. In a preferred embodiment, the phenolic resin is anovolac, a resole or a polyvinylphenol. The novolac is preferably acresol/formaldehyde or a cresol/xylenol/formaldehyde novolac, the amountof novolac advantageously being at least 50% by weight, preferably atleast 80% by weight, based in each case on the total weight of allbinders. The amount of the phenolic resin is advantageously from 40 to99.8% by weight, preferably from 70 to 99.4% by weight, particularlypreferably from 80 to 99% by weight, based in each case on the totalweight of the nonvolatile components of the coating.

The dissolution behavior of the phenolic resin in the developer can befine-tuned by optional solubility regulating components. Moreparticularly, development accelerators and development inhibitors can beused. These ingredients can be added to the layer which comprises thephenolic resin and/or to (an)other layer(s) of the coating.

Development accelerators are compounds which act as dissolutionpromoters because they are capable of increasing the dissolution rate ofthe phenolic resin. For example, cyclic acid anhydrides, phenols ororganic acids can be used in order to improve the aqueousdevelopability. Examples of the cyclic acid anhydride include phthalicanhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride,3,6-endoxy-4-tetrahydro-phthalic anhydride, tetrachlorophthalicanhydride, maleic anhydride, chloromaleic anhydride, alpha-phenylmaleicanhydride, succinic anhydride, and pyromellitic anhydride, as describedin U.S. Pat. No. 4,115,128. Examples of the phenols include bisphenol A,p-nitrophenol, p-ethoxyphenol, 2,4,4′-trihydroxybenzophenone,2,3,4-trihydroxybenzophenone, 4-hydroxybenzophenone,4,4′,4″-trihydroxytriphenylmethane, and4,4′,3″,4″-tetrahydroxy-3,5,3′,5′-tetramethyltriphenyl-methane, and thelike. Examples of the organic acids include sulfonic acids, sulfinicacids, alkylsulfuric acids, phosphonic acids, phosphates, and carboxylicacids, as described in, for example, JP-A Nos. 60-88,942 and 2-96,755.Specific examples of these organic acids include p-toluenesulfonic acid,dodecylbenzenesulfonic acid, p-toluenesulfinic acid, ethylsulfuric acid,phenylphosphonic acid, phenylphosphinic acid, phenyl phosphate, diphenylphosphate, benzoic acid, isophthalic acid, adipic acid, p-toluic acid,3,4-dimethoxybenzoic acid, 3,4,5-trimethoxybenzoic acid,3,4,5-trimethoxycinnamic acid, phthalic acid, terephthalic acid,4-cyclohexene-1,2-dicarboxylic acid, erucic acid, lauric acid,n-undecanoic acid, and ascorbic acid. The amount of the cyclic acidanhydride, phenol, or organic acid contained in the coating ispreferably in the range of 0.05 to 20% by weight.

In a preferred embodiment, the coating also contains developerresistance means, also called development inhibitors, i.e. one or moreingredients which are capable of delaying the dissolution of theunexposed areas during processing. The dissolution inhibiting effect ispreferably reduced by heating, so that the dissolution of the exposedareas is not delayed and a large dissolution differential betweenexposed and unexposed areas can thereby be obtained. Such developerresistance means can be added to a layer which comprises the phenolicresin or to another layer of the material.

The compounds described in e.g. EP-A 823 327 and WO97/39894 act asdissolution inhibitors due to interaction, e.g. by hydrogen bridgeformation, with the alkali-soluble binder(s) in the coating. Inhibitorsof this type typically comprise a hydrogen bridge forming group such asnitrogen atoms, onium groups, carbonyl (—CO—), sulfinyl (—SO—) orsulfonyl (—SO₂—) groups and a large hydrophobic moiety such as one ormore aromatic nuclei.

Other suitable inhibitors improve the developer resistance because theydelay the penetration of the aqueous alkaline developer into the layercomprising the phenolic resin. Such compounds can be present in thelayer itself, as described in e.g. EP-A 950 518, or in a developmentbarrier layer on top of said layer, as described in e.g. EP-A 864 420,EP-A 950 517, WO 99/21725 and WO 01/45958. In the positive workingembodiment, the barrier layer preferably comprises a polymeric materialwhich is insoluble in or impenetrable by the developer, e.g. acrylic(co-)polymers, polystyrene, styrene-acrylic copolymers, polyesters,polyamides, polyureas, polyurethanes, nitrocellulosics, epoxy resins andsilicones. In this embodiment, the solubility of the barrier layer inthe developer or the penetrability of the barrier layer by the developercan be increased by exposure to heat or infrared light.

Preferred examples of inhibitors of the latter type includewater-repellent polymers such as a polymer comprising siloxane and/orperfluoroalkyl units. In a typical embodiment, the precursor comprises abarrier layer which contains such a water-repellent polymer in asuitable amount between 0.5 and 25 mg/m², preferably between 0.5 and 15mg/m² and most preferably between 0.5 and 10 mg/m². Higher or loweramounts are also suitable, depending on the hydrophobic/oleophobiccharacter of the compound. When the water-repellent polymer is alsoink-repelling, e.g. in the case of polysiloxanes, higher amounts than 25mg/m² can result in poor ink-acceptance of the non-exposed areas. Anamount lower than 0.5 mg/m² on the other hand may lead to anunsatisfactory development resistance. The polysiloxane may be a linear,cyclic or complex cross-linked polymer or copolymer. The termpolysiloxane compound shall include any compound which contains morethan one siloxane group —Si(R,R′)—O—, wherein R and R′ are optionallysubstituted alkyl or aryl groups. Preferred siloxanes arephenylalkylsiloxanes and dialkylsiloxanes. The number of siloxane groupsin the (co)polymer is at least 2, preferably at least 10, morepreferably at least 20. It may be less than 100, preferably less than60. In another embodiment, the water-repellant polymer is ablock-copolymer or a graft-copolymer of a poly(alkylene oxide) and apolymer comprising siloxane and/or perfluoroalkyl units. A suitablecopolymer comprises about 15 to 25 siloxane units and 50 to 70alkyleneoxide groups. Preferred examples include copolymers comprisingphenylmethylsiloxane and/or dimethylsiloxane as well as ethylene oxideand/or propylene oxide, such as Tego Glide 410, Tego Wet 265, TegoProtect 5001 or Silikophen P50/X, all commercially available from TegoChemie, Essen, Germany. Such a copolymer acts as a surfactant which uponcoating, due to its bifunctional structure, tends to position itself atthe interface between the coating and air and thereby forms a separatetop layer even when applied as an ingredient of the same solution as thephenolic resin.

Simultaneously, such surfactants act as a spreading agent which improvesthe coating quality. Alternatively, the water-repellent polymer can beapplied in a second solution, coated on top of the layer which comprisesthe phenolic resin. In that embodiment, it may be advantageous to use asolvent in the second coating solution that is not capable of dissolvingthe ingredients present in the first layer so that a highly concentratedwater-repellent phase is obtained at the top of the material.

The coating preferably also contains a compound which absorbs infraredlight and converts the absorbed energy into heat. The IR absorbingcompound may be present in the same layer as the phenolic resin, in theoptional barrier layer discussed above or in an optional other layer.According to a highly preferred embodiment, the dye or pigment isconcentrated in or near the barrier layer, e.g. in an intermediate layerbetween the oleophilic and the barrier layer. According to thatembodiment, said intermediate layer comprises the IR absorbing compoundin an amount higher than the amount of IR absorbing compound in theoleophilic or in the barrier layer. The concentration of the IRabsorbing compound in the coating is typically between 0.25 and 10.0 wt.%, more preferably between 0.5 and 7.5 wt. %. Preferred IR absorbingcompounds are dyes such as cyanine and merocyanine dyes or pigments suchas carbon black. Examples of suitable IR absorbers are described in e.g.EP-As 823327, 978376, 1029667, 1053868, 1093934, WO 97/39894 and00/29214. A preferred compound is the following cyanine dye:

On order to protect the surface of the coating, in particular frommechanical damage, a protective layer may also optionally be applied.The protective layer generally comprises at least one water-solublepolymeric binder, such as polyvinyl alcohol, polyvinylpyrrolidone,partially hydrolyzed polyvinyl acetates, gelatin, carbohydrates orhydroxyethylcellulose, and can be produced in any known manner such asfrom an aqueous solution or dispersion which may, if required, containsmall amounts, i.e. less than 5% by weight, based on the total weight ofthe coating solvents for the protective layer, of organic solvents. Thethickness of the protective layer can suitably be any amount,advantageously up to 5.0 μm, preferably from 0.1 to 3.0 μm, particularlypreferably from 0.15 to 1.0 μm.

Optionally, the coating and more specifically the one or more layer(s)which comprise the phenolic resin, may further contain additionalingredients. Preferred ingredients are e.g. additional binders,especially sulfonamide and phthalimide groups containing polymers, toimprove the run length and chemical resistance of the plate. Examples ofsuch polymers are those described in EP-A 933682, EP-A 894622 and WO99/63407. Also colorants can be added such as dyes or pigments whichprovide a visible color to the coating and which remain in the coatingat unexposed areas so that a visible image is produced after exposureand processing. Typical examples of such contrast dyes are theamino-substituted tri- or diarylmethane dyes, e.g. crystal violet,methyl violet, victoria pure blue, flexoblau 630, basonlblau 640,auramine and malachite green.

For the preparation of the lithographic plate precursor, any knownmethod can be used. For example, the above ingredients can be dissolvedin a solvent mixture which does not react irreversibly with theingredients and which is preferably tailored to the intended coatingmethod, the layer thickness, the composition of the layer and the dryingconditions. Suitable solvents include ketones, such as methyl ethylketone (butanone), as well as chlorinated hydrocarbons, such astrichloroethylene or 1,1,1-trichloroethane, alcohols, such as methanol,ethanol or propanol, ethers, such as tetrahydrofuran, glycol-monoalkylethers, such as ethylene glycol monoalkyl ether, e.g.2-methoxy-1-propanol, or propylene glycol monoalkyl ether and esters,such as butyl acetate or propylene glycol monoalkyl ether acetate. It isalso possible to use a mixture which, for special purposes, mayadditionally contain solvents such as acetonitrile, dioxane,dimethylacetamide, dimethylsulfoxide or water.

The end-user can image-wise expose the lithographic printing plateprecursor directly with heat, e.g. by means of a thermal head, orindirectly by infrared light, preferably near infrared light. Theinfrared light is preferably converted into heat by an IR lightabsorbing compound as discussed above. The heat-sensitive lithographicprinting plate precursor of the present invention is preferably notsensitive to visible light, i.e. no substantial effect on thedissolution rate of the coating in the developer is induced by exposureto visible light. Most preferably, the coating is not sensitive toambient daylight, i.e. visible (400–750 nm) and near UV light (300–400nm) at an intensity and exposure time corresponding to normal workingconditions so that the material can be handled without the need for asafe light environment. “Not sensitive” to daylight shall mean that nosubstantial change of the dissolution rate of the coating in thedeveloper is induced by exposure to ambient daylight. In a preferreddaylight stable embodiment, the coating does not comprise photosensitiveingredients, such as (quinone)diazide or diazo(nium) compounds,photoacids, photoinitiators, sensitizers etc., which absorb the near UVand/or visible light that is present in sun light or office lighting andthereby change the solubility of the coating in exposed areas.

The printing plate precursor of the present invention can be exposed toinfrared light by means of e.g. LEDs or a laser. Most preferably, thelight used for the exposure is a laser emitting near infrared lighthaving a wavelength in the range from about 750 to about 1500 nm, suchas a semiconductor laser diode, a Nd:YAG or a Nd:YLF laser. The requiredlaser power depends on the sensitivity of the image-recording layer, thepixel dwell time of the laser beam, which is determined by the spotdiameter (typical value of modern plate-setters at 1/e² of maximumintensity: 10–25 μm), the scan speed and the resolution of the exposureapparatus (i.e. the number of addressable pixels per unit of lineardistance, often expressed in dots per inch or dpi; typical value:1000–4000 dpi).

Two types of laser-exposure apparatuses are commonly used: internal(ITD) and external drum (XTD) plate-setters. ITD plate-setters forthermal plates are typically characterized by a very high scan speed upto 500 m/sec and may require a laser power of several Watts. XTDplate-setters for thermal plates having a typical laser power from about200 mW to about 1 W operate at a lower scan speed, e.g. from 0.1 to 10m/sec.

The known plate-setters can be used as an off-press exposure apparatus,which offers the benefit of reduced press down-time. XTD plate-setterconfigurations can also be used for on-press exposure, offering thebenefit of immediate registration in a multi-color press. More technicaldetails of on-press exposure apparatuses are described in e.g. U.S. Pat.No. 5,174,205 and U.S. Pat. No. 5,163,368.

In the development step, the non-image areas of the coating are removedby immersion in a conventional aqueous alkaline developer, which may becombined with mechanical rubbing, e.g. by a rotating brush. Duringdevelopment, any water-soluble protective layer present is also removed.Silicate-based developers which have a ratio of silicon dioxide toalkali metal oxide of at least 1 are preferred to ensure that thealumina layer (if present) of the substrate is not damaged. Preferredalkali metal oxides include Na₂O and K₂O, and mixtures thereof. Inaddition to alkali metal silicates, the developer may optionally containfurther components, such as buffer substances, complexing agents,antifoams, organic solvents in small amounts, corrosion inhibitors,dyes, surfactants and/or hydrotropic agents as well known in the art.The development is preferably carried out at temperatures of from 20 to40° C. in automated processing units as customary in the art. Forregeneration, alkali metal silicate solutions having alkali metalcontents of from 0.6 to 2.0 mol/l can suitably be used. These solutionsmay have the same silica/alkali metal oxide ratio as the developer(generally, however, it is lower) and likewise optionally containfurther additives. The required amounts of regenerated material must betailored to the developing apparatuses used, daily plate throughputs,image areas, etc. and are in general from 1 to 50 ml per square meter ofrecording material. The addition can be regulated, for example, bymeasuring the conductivity as described in EP-A 0 556 690.

The plate precursor according to the invention can, if required, then bepost-treated with a suitable correcting agent or preservative as knownin the art. To increase the resistance of the finished printing plateand hence to extend the print run, the layer can be briefly heated toelevated temperatures (“baking”). As a result, the resistance of theprinting plate to washout agents, correction agents and UV-curableprinting inks also increases. Such a thermal post-treatment isdescribed, inter alia, in DE-A 14 47 963 and GB-A 1 154 749.

Besides the mentioned post-treatment, the processing of the plateprecursor may also comprise a rinsing step, a drying step and/or agumming step.

The printing plate thus obtained can be used for conventional, so-calledwet offset printing, in which ink and an aqueous dampening liquid issupplied to the plate. Another suitable printing method uses so-calledsingle-fluid ink without a dampening liquid. Single-fluid inks which aresuitable for use in the method of the present invention have beendescribed in U.S. Pat. No. 4,045,232; U.S. Pat. No. 4,981,517 and U.S.Pat. No. 6,140,392. In a most preferred embodiment, the single-fluid inkcomprises an ink phase, also called the hydrophobic or oleophilic phase,and a polyol phase as described in WO 00/32705.

EXAMPLES

The following composition was coated on a web of a conventional grainedand anodized aluminum support at a wet coating thickness of 26 μm and aspeed of 16 m/min:

methoxypropanol (Dowanol PM ™) 410.80 g  methyl ethyl ketone 266.03 g tetrahydrofuran 209.20 g  40.4 wt. % solution (Alnovol SPN 452 ™) ofnovolac 103.25 g  in Dowanol PM ™ 3,4,5-trimethoxy cinnamic acid 5.34 gdye IR-1 (formula shown above) 2.10 g Basonylblau 640 ™ (contrast dye)0.53 g TEGO Glide 265 ™ (10 wt. % solution of a 0.85 gpolyalkyleneoxide/polysiloxane surfactant) TEGO Glide 410 ™ (10 wt. %solution of a 2.12 g polyalkyleneoxide/polysiloxane surfactant)

The coating was dried with air having a temperature of 135° C. and thensubjected to an optional heating and a cooling step. During the heatingstep, air having the temperature indicated in Table 1 was blown onto thecoating during 1.2 s. In Examples 1 and 2, the hot air nozzles wereswitched off. Immediately thereafter, the back side of the web wascontacted with a metal cooling roller having the temperature indicatedin Table 1. With a cooling roller at 57° C., the temperature of theheated coating is reduced to a value below Tg very rapidly (>30° C./s).With a cooling roller at 75° C., the temperature interval around Tg ispassed at a much lower average cooling rate.

The materials were then imaged on a Creo Trendsetter 3244 (830 nm) atvarious energy density settings. The exposed plates were processed in anAgfa Autolith PN85 processor operating at a speed of 0.84 m/min usingAgfa Ozasol EP26 developer at 25° C. and finally gummed with Agfa OzasolRC795. The IR-sensitivity was defined as the minimum energy density thatis required to obtain a 50% light absorption, measured on the developedplate at the wavelength maximum of the dye, in areas which have beenexposed with a dot area of a 50% screen (@200 lpi). The sensitivity wasdetermined on fresh material and on material aged at ambient conditionsduring the number of days as indicated in Table 1.

TABLE 1 heating cooling air temp. roller sensitivity(mJ/cm²) after agingExample (° C.) temp. (° C.) 0 d. 5 d. 10 d. 15 d. 25 d. 1 (comp.) — 57 —26 60 80 107 2 (inv.) — 75 — 103 110 133 130 3 (inv.) 170 75 93 160 187208 205

The data in Table 1 demonstrate that the materials which were cooledaccording to the invention, reach a stable sensitivity from 15 daysafter coating, whereas the sensitivity of the comparative material inExample 1 was still shifting after 25 days and was found to level off at165 mJ/cm² about two months after coating.

The improved aging behavior can also be demonstrated by subjecting thematerials to an off-line heat treatment after applying the method of thepresent invention. Examples 4, 5 and 6 were obtained by coating the samecomposition and drying according to the same procedure as discussed forthe previous examples. The dried materials were then subjected to a heattreatment by using an additional drier which blew air at 135° C. ontothe dry coating. Between the latter heat treatment and the coolingroller, the web cooled under ambient conditions so that the webtemperature was 118° C. just before the cooling roller. A first rapidcooling was obtained with a metal roller of which the temperature wasmaintained at the value given in Table 2. The contact between thematerial and the cooling roller lasted 2.41 s. Immediately after passingthe metal cooling roller, a second slower cooling phase was passed byblowing air at 50° C. onto the coating during 32 s.

TABLE 2 Cooling at metal roller (2.41 s) Cooling with air web temp.average @50° C. (32 s) cooling just after cooling rate web temp. averageroller cooling at cooling just after cooling rate temp. roller rollercooling in cooling Example (° C.) (° C.) (° C./s) air (° C.) air (°C./s) 4 65 66 22 51 0.47 5 75 76 18 57 0.59 6 85 86 14 63 0.72

The sensitivity was measured on fresh material and on material that wasartificially aged off-line during 7 days at the temperature given inTable 3. The data in Table 3 demonstrate that the materials which arecooled more slowly, i.e. with a higher cooling roller temperature, havea more stable aging behavior. Example 6 was cooled in the interval nearTg at a very slow cooling rate and provides the best aging behavior,since there is little effect from the additional off-line heattreatment.

TABLE 3 Example 0 d. 7 d. @20° C. 7 d. @25° C. 7 d. @30° C. 4 17 159 210222 5 87 167 200 226 6 101 210 220 233

1. A method of making a heat-sensitive lithographic printing plateprecursor comprising the steps of (i) providing a web of a lithographicsupport having a hydrophilic surface; (ii) applying a coating comprisinga phenolic resin on the hydrophilic surface of the web; (iii) drying thecoating by supplying heat to the coated web; (iv) an active cooling stepwherein the web temperature is reduced at an average cooling rate whichis higher than if the web would be kept under ambient conditions andvaries between 0.5 and 30° C./s; and (v) winding the precursor on a coreor cutting the precursor into sheets.
 2. The meted according to claim 1wherein the average cooling rate is not higher than 20° C./s.
 3. Themethod according to claim 2 further comprising a heating step betweenstep (iii) and step (iv), wherein during said heating step the webtemperature is maintained above the glass transition temperature of thephenolic resin during a period of between 0.1 and 60 seconds.
 4. Themethod according to claim 3 wherein during the cooling step the webtemperature is reduced in a first phase down to T1 at an average coolingrate of at least 10° C./s; and in a second phase from T1 to T2 at anaverage cooling rate which is lower than 10° C./s.
 5. The methodaccording to claim 3 wherein dining the cooling step the web temperatureis reduced in a first phase dawn to T1 at an average cooling rate of atleast 10° C./s; and in a second phase from T1 to T2 at an averagecooling rate which is lower than 10° C./s; and in a third phase from T2to about ambient temperature at an average cooling rate of at least 10°C./s.
 6. The method according to claim 1 wherein the average coolingrate is not higher than 10°C./s.
 7. The method according to claim 6further comprising a heating step between step (iii) and step (iv),wherein during said heating step the web temperature is maintained abovethe glass transition temperature of the phenolic resin during a periodof between 0.1 and 60 seconds.
 8. The method according to claim 7wherein during the cooling step the web temperature is reduced in afirst phase down to T1 at an average cooling rate of at least 10° C./s;and in a second phase from T1 to T2 at an avenge cooling rate which islower than 10° C./s.
 9. The method according to claim 7 wherein duringthe cooling step the web temperature is reduced in a first phase down toT1 at an avenge cooling rate of at least 10° C./s; and in a second phasefrom T1 to T2 at an average cooling rate which is lower than 10° C./s;and in a third phase from T2 to about ambient temperature at an averagecooling rate of at least 10° C./s.
 10. The method according to claim 1wherein at the beginning of the cooling step the web temperature ishigher than Tg, the glass transition temperature of the coatingcomprising the phenolic resin, and wherein during the cooling step theweb temperature is reduced from T1 to T2, T1 being higher than Tg and T2being lower than Tg, at an average cooling rate which is lower than 10°C./s.
 11. The method according to claim 10 wherein during the coolingstep the web temperature is reduced in a first phase down to T1 at anaverage cooling rate of at least 10° C./s; and in a second phase from T1to T2 at an average cooling rate which is lower than 10° C./s.
 12. Themethod according to claim 11 wherein the cooling from T1 to T2 proceedsat an average cooling rate which is lower than 5° C./s.
 13. The methodaccording to claim 12 further comprising a heating step between step(iii) and step (iv), wherein dining said heating step the webtemperature is maintained above the glass transition temperature of thephenolic resin during a period of between 0.1 and 60 seconds.
 14. Themethod according to claim 11 wherein T1 is Tg+20° C. and T2 is Tg−20° C.15. The method according to claim 14 further comprising a heating stepbetween step (iii) and step (iv), wherein during said heating step theweb temperature is maintained above the glass transition temperature ofthe phenolic resin during a period of between 0.1 and 60 seconds. 16.The method according to claim 11 wherein T1 is Tg+10° C. and T2 isTg−10° C.
 17. The method according to claim 16 further comprising aheating step between step (iii) and step (iv), wherein during saidheating step the web temperature is maintained above the glasstransition temperature of the phenolic resin during a period of between0.1 and 60 seconds.
 18. The method according to claim 11 furthercomprising a heating step between step (iii) and step (iv), whereinduring said heating step the web temperature is maintained above theglass transition temperature of the phenolic resin during a period ofbetween 0.1 and 60 seconds.
 19. The method according to claim 10 whereinduring the cooling step the web temperature is reduced in first phasedown to T1 at an average cooling rate of at least 10° C./s; and in asecond phase from T1 to T2 at an average cooling rate which is lowerthan 10° C./s; and in a third phase from T2 to about ambient temperatureat an average cooling rate of at least 10° C./s.
 20. The methodaccording to claim 19 wherein the cooling from T1 to T2 proceeds at anavenge cooling rate which is lower than 5° C./s.
 21. The methodaccording to claim 20 further comprising a heating step between step(iii) and step (iv), wherein during said heating step the webtemperature is maintained above the glass transition temperature of thephenolic resin during a period of between 0.1 and 60 seconds.
 22. Themethod according to claim 19 wherein T1 is Tg+20° C. and T2 is Tg−20° C.23. The method according to claim 19 wherein T1 is Tg+10° C. and T2 isTg−10° C.
 24. The method according to claim 19 further comprising aheating step between step (iii) and step (iv), wherein during saidheating step the web temperature is maintained above the glasstransition temperature of the phenolic resin during a period of between0.1 and 60 seconds.
 25. The method according to claim 10 wherein thecooling from T1 to T2 proceeds at an average cooling rate which is lowerthan 5° C./s.
 26. The method according to claim 25 wherein T1 is Tg+20°C. and T2 is Tg−20° C.
 27. The method according to claim 25 wherein T1is Tg+10° C. and T2 is Tg−10° C.
 28. The method according to claim 25further comprising a heating step between step (iii) and step (iv),wherein during said heating step the web temperature is maintained abovethe glass transition temperature of the phenolic resin during a periodof between 0.1 and 60 seconds.
 29. The method according to claim 10wherein T1 is Tg+20° C. and T2 is Tg−20° C.
 30. The method according toclaim 29 further comprising a heating step between step (iii) and step(iv), wherein during said heating step the web temperature is maintainedabove the glass transition temperature of the phenolic resin during aperiod of between 0.1 and 60 seconds.
 31. The method according to claim10 wherein T1 is Tg+10° C. and T2 is Tg−10° C.
 32. The method accordingto claim 31 further comprising a heating step between step (iii) andstep (iv), wherein during said heating step the web temperature ismaintained above the glass transition temperature of the phenolic resinduring a period of between 0.1 and 60 seconds.
 33. The method accordingto claim 10 further comprising a heating step between step (iii) andstep (iv), wherein during said heating step the web temperature ismaintained above the glass transition temperature of the phenolic resinduring a period of between 0.1 and 60 seconds.
 34. The method accordingto claim 1 further comprising a heating step between step (iii) and step(iv), wherein during said heating step the web temperature is maintainedabove the glass transition temperature of the phenolic resin during aperiod of between 0.1 and 60 seconds.